1. Field of the Invention
This invention generally relates to analog-to-digital converters (ADCs) and, more particularly, to a system and method for correcting errors in an interleaved ADC.
2. Description of the Related Art
An N-path time interleaved ADC consists of N component ADCs operated in parallel, which together sample the signal N times at the rate of the individual ADC components. In practice, the component ADCs are never truly identical, and the sampling clocks they receive can have small phase deviations from the ideal sampling phase. As a result, these timing and gain errors produce artifacts that in frequency domain show up as spectral images of the desired signal centered around every multiple of fs/N, where fs is the sampling rate of the composite ADC. If the errors are known they can be corrected with either digital post-processing after the ADC, or with an analog correction circuitry in the ADC, or with some combination of the two. However, without knowing the ADC input signal, error detection is difficult.
One way to facilitate the error detection task is to inject a narrow band known test signal into the ADC input. One prior art solution uses a high pass filter that removes most of the other signal content preserving the spectral image of the test tone, based on the assumption that the spectrum around the tone is relatively clean from noise and other signals. This is not necessarily the case in a real world application. For instance, if the tone to be detected is near fs/2 of the ADC bandwidth, this region of spectrum typically falls in the transition band of the anti-alias filter that is often used in front of the ADC, and it can have partially attenuated spectra of neighboring signal channels or other unwanted signals. The accuracy of the tone detection is limited by this additional signal content in the pass band of the filter. The filter is also fairly costly to implement due to its relatively narrow bandwidth.
U.S. Pat. No. 7,932,845 describes an error detection method that performs a fast Fourier transform (FFT) on individual ADC outputs before they are interleaved. If a wideband input signal is introduced, the individual ADC outputs have aliased signal spectra that can “hide” a small test tone being introduced for the purpose of detection and correction. As a result, this method does not provide a useful means of correcting errors in the presence of an input signal, and does not provide true background calibration.
It would be advantageous if errors in an interleaving ADC could be accurately detected and minimized using a narrowband background test tone in the presence of an input signal.